This invention relates to high power (nonlinear) testing of microwave transistors (DUT) in the frequency and time domain. When said transistor is driven in its nonlinear operation regime, a sinusoidal input electrical signal is deformed and the output signal contains harmonic components, following Fourier's analysis of periodic non-sinusoidal quantities. The electrical signals injected at the input of the DUT and extracted at the output can be sampled using signal sampling devices, such as signal couplers [1], wave-probes [5] or IV probes [3] and processed by appropriate signal analyzers [7].